The present invention relates to an electron gun for a color cathode ray tube, and more particularly to a dynamic focus electron gun capable of forming beam spots with small halos on the periphery of a screen and beam spots of regular size on both the center and periphery of the screen.
The resolution of a color cathode ray tube greatly depends on the characteristic of electron beam spots formed on a screen. To obtain an image of good quality, the electron beam spot formed on the screen should be as small as possible with the smallest halo around its core. Of course, it is desirable for the beam spot to experience as little distortion as possible. However, since conventional RGB electron guns are arranged in-line and a deflection yoke is adopted which forms a pincushion horizontal deflection magnetic field and a barrel vertical deflection magnetic field, electron beam spots formed on the periphery of the screen become distorted due to the influence of astigmatism while electron beams pass through an uneven magnetic field formed by the deflection yoke.
In other words, when electron beams land on the center of a screen, where the deflection magnetic field does not affect the beams, astigmatism of the electron beams does not occur, and a circular electron beam spot without halo is formed. However, when deflecting toward the periphery of the screen, owing to a strong deflection magnetic field, the electron beams diverge in the horizontal direction and are excessively focused in the vertical direction, so that electron beam spots having a bright core and a dim halo are formed on the screen.
One example of an electron gun for a conventional color cathode ray tube designed to improve the above-described problem is illustrated in FIG. 1.
This electron gun includes a triode for producing an electron beam consisting of a cathode 2, a control electrode 3 and a screen electrode 4, and a major lens for accelerating and focusing the electron beam consisting of a static focus electrode 5 adjacent to screen electrode 4, a dynamic focus electrode 6 and a final accelerating electrode 7.
Vertically-elongated electron beam passing hole 5H and horizontally-elongated electron beam passing hole 6H are respectively formed in the electron beam passing planes of static focus electrode 5 and dynamic focus electrode 6 which face each other. Static focus electrode 5 is supplied with a predetermined static focus voltage Vf. Final accelerating electrode 7 is supplied with an anode voltage Ve being higher than focus voltage Vf. Dynamic focus electrode 6 is supplied with a dynamic focus voltage Vd which is synchronized with deflection signals and its negative peak equals focus voltage Vf.
A reference numeral 100 is a magnetic lens which represents the uneven magnetic field of the deflection yoke by means of an optical lens.
In the above-described electron gun, when the electron beam is not deflected, in other words, when the electron beam emitted from the electron gun scans the center of the screen, dynamic focus voltage Vd whose negative peak voltage equals focus voltage Vf is supplied to dynamic focus electrode 6. Therefore, a lens capable of controlling the electron beam is not formed between static and dynamic focus electrodes 5 and 6. Thus, the electron beam maintains an unaffected circular shape when passing static and dynamic focus electrodes 5 and 6, and a nearly circular beam spot is formed on the screen.
Meanwhile, when the electron beams emitted from cathode 2 scans the periphery of the screen, dynamic focus voltage Vd being higher than static focus voltage Vf supplied to static focus electrode 5 is applied to dynamic focus electrode 6, so that an electron lens, particularly a quadrupole lens 56, is formed between focus electrode 5 and dynamic focus electrode 6. This quadrupole lens 56 is composed of a first lens element 56a which has a diverging force in the vertical direction and a second lens element 56b which has a focusing force in the horizontal direction, due to the vertically-elongated electron beam passing hole 5H formed in the outgoing plane of static focus electrode 5 and the horizontally-elongated electron beam passing hole 6H formed in the incoming plane of dynamic focus electrode 6. Accordingly, the electron beam diverges in the vertical direction and focuses in the horizontal direction while passing through quadrupole lens 56, thereby being vertically elongated. Then, the narrow width in the horizontal direction of the vertically elongated electron beam is compensated by compensating for defocusing due to the vertical excessive focusing by the uneven magnetic field, so that a beam spot without halo can be obtained on the screen.
In the conventional dynamic focus electron gun, since dynamic focus voltage Vd is higher than static focus voltage Vf at the center of the screen, an extremely high dynamic focus voltage Vd must be supplied to eliminate the halo along the diagonal lines of the screen. However, it is difficult to realize a driving circuit for supplying voltages to each electrode of the triode. Moreover, the withstand voltage characteristic of the electron gun is deteriorated.
Furthermore, in the electron gun, although occurrence of a halo at the periphery of the screen can be suppressed by the quadrupole lens, a compensation effect on the cross-sectional shape of the electron beam caused by the deflection magnetic field of the deflection yoke is incomplete. For this reason, distortion of the electron beam spot cannot be sufficiently compensated which makes the size of the vertical beam spot smaller than the distance between apertures of the shadow mask, and a moire effect occurs on the screen when the vertical diameter of the beam spot is not more than twice the distance between apertures of the shadow mask.